Process for manufacturing acrylic esters and other unsaturated compounds by thermal decomposition



Patented June 19,1945

ESTERS AND OTHER UNSATURATED COM- POUNDS BY THERMAL DECOMPOSITION William P. Ratchford, Willow Grove, and Charles H. Fisher, Abington, Pa., assignors to Claude R. Wickard, as Secretary of Agriculture of the United States of America No Drawing. Application August 4, 1943,

Serial No. 497,344

3 Claims. (Cl. 260-486)v (Granted under the act or March 3, 1883, as

amended April 30, 1928; 370 0. G. 757) This application is made under the .act of March 3, 1883, as amended by the act of April 30, 1928, and the invention herein described, if patented, may be manufactured and used by or for the Government of the .United States of America for governmental purposes without the payment to us of any royalty thereon.

This invention relates to the production of unsaturated compounds by the thermal decomposition of carboxylic acid esters, and more particularly to the manufacture of acrylic esters by passing vapors of lactic acid derivatives through pyrolysis tubes vated temperatures.

It is known that acrylic esters are formed when certain lactic acid derivatives. such as methyl or chambers maintained at 816- alpha-acetoxypropionate, are passed through tubes maintained at temperatures ranging from 400 to 600 C. (Ritchie, Jones and Burns, U. S. 2,265,814, December 9, 1941; Smith and coworkers, Ind. Eng. Chem. 34, 473 (1942).) Moreover, it is known that esters of carboxylic acids generally decompose on pyrolysis into carboxylic acids, olefins, diolefins, and other unsaturated compounds. A diiiiculty attending the use of these pyrogenic methods is that, when certain metals and alloys are used as contact and construction materials, corrosion occurs and undesirable by-products are formed.

An object of this invention is to provide a commercial process for manufacturing various unsaturated compounds, including esters such as acrylic esters, by passing esters, including lactic esters, through a hot tube or pyrolysis chamber.

A further object is to provide inexpensive and readily availableconstruction materials for the pyrolysis tube or chamber which will increase conversion per pass, increase throughput for a given plant, and decrease productioncosts and capital investment.

A further object is to provide a method for inactivating or modifying contact and construction materials so that undesirable by-products, particularly gases and carbonaceous materials, are formed in only small and negligible quantities. I Other objects will appear from the following description.

The conversion of carboxylic acid esters into carboxylic acid and unsaturated compounds is highly dependent upon the nature of the packing of the hot tube and the nature of the material used in construction of the pyrolysis chamber. Stainless steel of the 18-8 type (18 percent chromium and 8 percent nickel) is a suitable construction material, but it is relatively expensive and inaccessible. Ordinary iron and steel tubes and pipes are more readily available, but these materials catalyze or cause. the formation of undesired by-products, chiefly gases and carbonaceous materials, during the pyrolysis.

We have found that thisdeleterious effect, that is, the formation of undesired by-products, can be decreased by the use of inhibitors or materials which diminish or destroy the catalytic activity.

Thus, we have found that the undesired catalytic effect of iron and its alloys can be diminished by the use of inhibitors, such as oxygen, sulfur, and nitrogen compounds. The invention described" herein makes possible th use of iron and iron alloy chambers Or tubes in the pyrolysi 'of esters, decreases the yield of undesirable byproducts, and increases the yield of the desired unsaturated compounds and organic acid.

Our invention is'illustrated by the following experimental procedure: a

The lactic acid derivative, methyl alpha-acetoxypropionate, was pyrolyzed under many different conditions in equipment constructed of ordinary iron. The diameter of the iron pipe was one inch and the heated portion was 12 inches long. The temperature, which was controlled automatically, was measured by a thermocouple located in a well extending through the center of the iron pipe.

The pyrolysis experiments showed that iron catalyzes or causes the formation of normally gaseous products and carbonaceous materials, particularly at the higher temperatures. At any given temperature the catalytic formation of byproducts increased with time. For example, at

545 C. and a contact time of '15 seconds, ap-

proximately 50 percent of the methyl alpha-acetoxypropionate was converted into products which are gases at normal temperature and pressure. Under these conditions, the yields of methyl acrylate and acetic acid are very low. This un-,

desired catalytic activity of the iron, however, was virtually eliminated by passing steam and air through the pyrolysis equipment for a short time while the temperature was kept at 525 C.

' acrylate and acetic acid were obtained in high yields by the thermal decomposition of methyl alpha-acetoxypropionate.

In another case, inactivation of the iron was caused by dissolving water in the methyl acetoxypropionate prior to the pyrolysis.

We are not limited to the particular materials or procedures disclosed above. The general applicability of our process to the production of esters of this type will readily be seen by those skilled in the art. For example, water may be dissolved in the ester to be pyrolyzed and the mixture passed into the pyrolysis unit, or the ester and water, as liquid or vapor, may be passed individually into the pyrolysis unit. Materials such as tertiary butyl alcohol, methyl hydracrylate, diacetone alcohol and hydrogen peroxide which readily d compose into water or oxygen.

also may be passed into the pyrolysis unit to prevent or decrease the formation of undesirable by-products. Oxygen analogs such as hydrogen sulfide, mercaptans, sulfur, ammonia, amines about 400 to 600 C. in pyrolysis equipment having iron as the principal ingredient, the step which comprises treating the metal surfaces in the reaction zone at a temperature of the order of the pyrolysis temperature with a mixture of steam and air immediately prior to pyrolyzing the methyl -alpha-acetoxypropionate.

2. In the process of manufacturing methyl aorylate by the thermal decomposition of methyl alpha-acetoxypropionate at a temperature of about 400 to 600 C. in pyrolysis equipment having iron as its principal ingredient, the step which comprises treating the metal surface in the reaction zone at a temperature of the order of the pyrolysis temperature with water vapor immediately prior to pyrolyzing the methyl alpha-acetoxypropionate. v

3. In the process of manufacturing methyl acrylate by the thermal decomposition of methyl alpha-acetoxypropionate at a temperature of about 400 to 600 C. in pyrolysis equipment having iron as its principal ingredient, the steps which comprise mixin the methyl alphaacetoxypropionate with water, and then pyrolyzing said mixture.

WILLIAM P. RATCHFORD. CHARLES H. FISHER. 

